-I dir, --include-dir=dir
Specifies a directory dir to search for included makefiles. If
several -I options are used to specify several directories, the
directories are searched in the order specified. Unlike the argu‐
ments to other flags of make, directories given with -I flags may
come directly after the flag: -Idir is allowed, as well as -I dir.
This syntax is allowed for compatibility with the C preprocessor's
-I flag.